An ignition plug is mounted in an internal combustion engine (an engine) and is used to ignite an air-fuel mixture or the like in a combustion chamber. In general, the ignition plug includes an insulator having an axial hole which extends in an axial direction, a center electrode which is inserted in a front end side of the axial hole, a metal shell which is provided on an outer circumference of the insulator, and a ground electrode which is fixed to a front end portion of the metal shell. In addition, the insulator is fixed to the metal shell in such a state that a step portion provided on an outer circumference of the insulator engages with an inner circumference of the metal shell directly or via a metallic plate packing Then, when the internal combustion engine is in operation, heat received by a front end portion of the insulator is drawn mainly from the step portion towards the metal shell.
Further, a spark discharging gap is defined between a distal end portion of the ground electrode and a front end portion of the center electrode. By applying a high voltage to the spark discharging gap to thereby generate a spark discharge, the air-fuel mixture is ignited (refer to JP-A-2007-242588, for example). Along with this, in order to suppress generation of radio noise in association with the application of the high voltage, a resistor containing metal and glass is provided further rearwards than the center electrode in the axial hole (that is, along an energization path of the spark discharging gap) in a position lying further rearwards than the step portion.
Incidentally, in recent years, in order to cope with the demand for improved fuel economies and regulations to preserve the environment, there have been proposed engines which are highly supercharged and engines with high compression ratios. In these engines, since a relatively high pressure is generated in a combustion chamber, a relatively high voltage is necessary to generate a spark discharge (a spark discharging voltage). If the spark discharging voltage is increased, a spark discharge which penetrates the insulator (a through discharge) is generated in a location lying further forwards towards the front end side than a location where the insulator contacts the metal shell or the plate packing (namely, at a thinner location in particular). Accordingly, a normal spark discharge may be disturbed (an accidental fire is caused).
Taking this into account, it is considered to improve the withstand voltage performance of the insulator by increasing the density (the relative density) of the insulator so as to suppress the generation of the through discharge. Here, in the related-art ignition plug, a relative density of the insulator is uniform in every portion on the insulator.